The reverse transcriptase inhibitor 3'-azido-deoxythymidine (AZT) has previously been shown to be incorporated into specific regions near the telomeres and centromeres of Chinese hamster ovary cell chromosomes. Our investigation of the effects of AZT on chromosome stability has led to the discovery of a high frequency amplification of telomere-like centromeric DNA. The amplified structures, when analyzed cytogenetically, appear as tandem arrays of tightly clustered blocks of centromeric repeats containing telomeric sequences (TTAGGG)n. There were 5-13 blocks of amplified DNA per structure. These structures form rapidly within one or two cell cycles and can be observed with an incidence as high as 2%. Because the amplification was so rapid, we tested whether the amplification structures could be the result of aberrant overreplication by analyzing BrdU incorporation. Our results indicate that the amplified DNA does not undergo abnormal replication during its formation, but appears to form from existing centromeric regions. We propose a model that involves the excision of multiple centromeric DNA regions from other chromosomes and their relocalization to a new site.